A common industrial problem involves the cutting of tubular shape objects at points that are inaccessible, impractical, or unsafe using conventional torches or other human-held cutting tools. An example of this problem involves the demolition of offshore oil and gas production platforms. The legs of these production platforms are typically made from large diameter tubular steel members that are partially embedded on one end into the sea floor. These tubular legs then extend upward through the water to the surface where the external production structure is attached. These tubular legs serve both to anchor the platform to a fixed point and to support the super-structure at the surface where the oil and gas recovery and processing operations are performed. When the platform is demolished, the operations structure is first removed and the remaining platform and tubular legs must be subsequently disconnected from the sea floor. For complete removal of navigation hazards and elimination of hazards to the fishing and shrimping industries, it is necessary to comply with government requirements that the tubular legs be severed at a point below the sea floor level or “mud-line.” The preferred method is to sever the tubular members using explosives below the mudline from the inside of the tubular member. It is desirable to use the smallest amount of explosives possible to achieve severance of the tubular members and not disturb the surrounding marine environment.
Most tubular members used in the oil and gas platform construction are generally comprised of short sections of pipe connected together by special load-bearing connectors. Therefore, the tubular member provides an open annulus where jetting and vacuum lines could be inserted and used to remove the mud to a point below the natural sea floor. One particular problem to placing and operating lines and cutting tools within the annulus results from the load-bearing connectors, which form internal constrictions at the various joining points within the interior of the length of pipe. Lines and cutting equipment must be able to navigate these constrictions without becoming snagged or jammed.
Many different types of cutting equipment are available, such as water jet cutters, underwater torches, underwater cutting wheels, etc. However, the most popular and cost effective means to cut these tubular members involves the use of commercially available linear explosive shape charges. Linear explosive shape charges, which use small amount of explosives to produce a directed cutting blaze, are particularly well suited for such applications and are comprised of elongated masses of explosive material having V-shape cross-sections. These types of charges must be positioned in a nearly continuous circle within the tube adjacent to the wall in order to achieve the complete circumferential severance of the tubular member. Upon detonation of such linear shape charges, because of the housing material and shape of the explosive, a substantial unidirectional explosive jet of high temperature and velocity is produced capable of deep penetration of metal thicknesses. However, it is essential that an air space exist between the shape charge and the target to be severed in that the jet produced must travel a distance before meeting incompressible liquids or other obstructions in order to achieve proper penetration. The length of the air space required between the shape charge and the target to achieve proper penetration of the target is known in the art as the “stand-off” distance. The requirement of placing the shape charge in a chamber that will provide this air space that can be transported to the depth needed and then expanded so that the chamber is within close proximity to the tubular wall is necessary to have a successful severance of the tubular wall.